The present invention relates to furnaces. In particular, it relates to furnaces of the kind in which a toroidal fluid flow heating zone is established. Such furnaces are described, for example, in U.S. Pat. No. 4,479,920.
Generally, a hot gas is passed through gaps between angled blades or vanes in a ring of blades or vanes provided in an operational chamber of the furnace. The blade ring is formed in an annular gap between the wall of the chamber and a central block, for example, an upwardly pointing conical portion, located on the axis of the chamber. Gas flow is caused to follow a rotary path in a doughnut shaped region around the block and in individual swirls within the rotary path. This ensures efficient residence of, and heat transfer to, material, such as particulate material, to be heated in the gas flow.